As will be appreciated by those skilled in the art, the wireless or mobile telephone is no longer simply used for voice communications. Although voice communications remain a primary purpose for many of these communication devices, these devices are being called on to provide the same communication data exchange services demanded by ground based fixed location or hard path communication systems (optical and/or wire) including the transmission of vast amounts of data such as provided through public and private internet connections. The mobility of the wireless units and the high data transmission rates used with these systems presents challenges and problems never before experienced by ground based fixed location systems.
CDMA cellular network communication systems, such as W-CDMA (Wideband Code Division Multiple Access) for UMTS and CDMA 2000 systems are among the most commonly deployed wireless technologies. In CDMA systems, the mobile station uses the searcher circuitry primarily for cell measurement and delay profile estimation. Firstly, the cell measurement refers to the mobile's operation of measuring the radio link quality of its neighbor cells as well as its active cells in order to support various types of handover. As a mobile station moves from one cell's coverage area to another, a signal quality from the original base station or active cells degrades while a signal from a neighbor base station becomes better than that from the original base station. The handoff or handover refers to the process in which the mobile station switches its communication link from the original cell to the new cell under the network's approval. To support the handoff process, the network gives the mobile station a list of the neighbor cells and requests that the mobile station monitor them. As may be recognized by those skilled in the art, a large number of neighbor cells makes cell deployment easier but demands a high searcher throughput from the mobile station. On the one hand, the mobile station must monitor the neighbor cells diligently in preparation for a possible handoff to prevent loss of the communication link. On the other hand, the mobile station must avoid overusing its battery power for the searcher when the surrounding environment is much more benign (fewer cells and/or received signals) than the worst-case scenario for which the searcher throughput is designed. Secondly, the mobile station measures the delay profile of a signal from the active cell with which the mobile station exchanges information on the communication link. The delay profile is one of the properties of the wireless propagation channels that change dynamically over time. In the wireless channel, buildings and trees scatter an electromagnetic wave and generate a multiplicity of “paths” via which an electromagnetic wave transmitted from the antenna of the active cell arrives at the antenna of the mobile station. The propagation delay of each path is different and most mobile receivers rely on the accurate estimation of the profile of the multipath propagation delays. For example, a Rake receiver needs to estimate the propagation delay of each individual path of the electromagnetic wave from the transmission antenna of the active cell in order to allocate its demodulator element or finger. The mobile station must frequently monitor the multipath delay profile of the existing link(s) so that its “fingers” (parallel demodulation elements of the Rake receiver) can be assigned to newly-found strong multipath signals and so that any degrading multipath signals can be unassigned or removed. At the same time, the mobile station must also avoid overusing its battery power for the searcher when the surrounding environment is much more benign (slowly changing propagation channel) than the worst-case scenario for which the searcher throughput is designed.
Therefore, it is seen that a method and apparatus for scheduling cell searches in order to efficiently use the searcher hardware of the mobile unit for searching a multiplicity of cells is needed. Searching the various cells should take into consideration the searcher hardware capability and the search requirements based on the environment. This method will reduce the power consumption without degrading the overall receiver performance.
According to one prior art method, the mobile station searches the neighbor cells in a fixed pattern (e.g., cell1, cell2, cell3, . . . ) repeatedly regardless of the number of cells. This fixed pattern search method is easy to implement, but has two problems. First, if the number of neighboring cells increases over time, the fixed pattern search method may not be able to meet the search rate requirement. Second, according to this prior art method, the searcher always runs at full speed even when there are only a few cells. Thus the number of searches per cell is excessive and unnecessary. These unnecessary searches consume power and of course shorten battery life.
According to another prior art method, a number of predetermined fixed search patterns are employed to dynamically adjust the search rate according to the number of cells. This method requires the mobile station to store the various search patterns for each scenario. The flexibility of this method is superior to the single fixed pattern discussed above, but is limited by the number of search patterns.
According to a third prior art method, the mobile station runs the searcher at a constant rate, such as for example, searching one cell every 6 ms. This method also dedicates one third of the searcher throughput to the active cell(s). This method is simple but difficult to use for a larger number of cells as in the case of W-CDMA and is not designed for a fast search based on vector coordinates.
In summary, new apparatus and methods are needed to improve the capacity, fidelity, and performance of wireless or mobile digital communication. More specifically, a need arises for apparatus and methods to improve the searching efficiency of the active cell(s) and neighboring cells by a mobile phone while conserving power. In particular, a need arises for a method to select for searching the most worthwhile candidates from all the different neighbor cells received by a mobile phone for a subsequent handoff and/or demodulation and combining operation. Additionally, a need arises for a method to select the most likely signals for subsequent handoff while at the same time conserving battery power.